/opt/stack/os-config-refresh is the default base directory. You can
set OS_REFRESH_CONFIG_BASE_DIR environment variable to override the
default one.

Its intended purpose is to separate scripts execution into 4 phases:

Quiesce(pre-configure.d),

Configure(configure.d),

Activate(post-configure.d).

Migrate(migration.d),

It runs through all the phases above to ensure configuration is
applied and enabled on a machine. It will exit with an error if any
phase has a problem. The scripts in each phase should not depend on
each other having worked properly.

Note: Earlier versions of os-refresh-config ran migration before
post-configure. This was an oversight in the initial design, as
migrations are intended to be online migrations after the host is
fully configured.

For things which must happen while the service is quiesced, that should
be done in the post-configure scripts which control the service state.